Adaptation of Anopheles coluzzii Larvae to Polluted Breeding Sites in Cotonou: A Strengthening in Urban Malaria Transmission in Benin

Objectives: Urban malaria is on the rise in Cotonou due to rapid urbanization and development activities. However, malaria transmission depends on several factors. Hydrology is a key factor that influences the presence of suitable habitats for the development of Anopheles larvae, thereby affecting survival of the malaria vector. For a long time, clean water bodies were thought to be the favorite Anopheles gambiae s.l. breeding sites. Nowadays, Anopheles gambiae s.l. is found in polluted waters, which used to be only favorable for the development of Culex quinquefaciatus. The current study was designed to assess both the water quality of Anopheles breeding sites and the ability of Anopheles larvae to develop in polluted water bodies. Methods: Larvae were collected in four districts in the city of Cotonou. All open water bodies were considered as potential breeding sites and investigated. Most of the breeding sites were shallow, exposed to sunlight and contained different qualities of water. Four breeding sites containing Anopheles larvae were identified and sampled. Two water samples were taken from each breeding site, in 1.5 L and 10 L containers for the measurement of physicochemical parameters and the monitoring of larval development, respectively. Results: Fourteen physicochemical parameters were measured, and all four breeding sites sampled had a very high level of pollution with high values of temperature, color, chemical oxygen demand, biological oxygen demand, turbidity, total phosphors and suspended particles. Laboratory breeding of A. coluzzii larvae collected from field using polluted water samples, has yielded a high pupae rate (90%) which was significantly different from that observed with a laboratory strain of A. gambiae Kisumu (78,67%). Conclusion: These results indicate that the adaptation of A. coluzzii to polluted waters in Cotonou is quite alarming especially for urban malaria control.